The present invention relates to devices for spacing the turns of thread for units for feeding weft to textile machines, particularly weaving looms.
More specifically, the present invention relates to weft feeders which comprise a fixed drum, on which a windmilling hollow arm winds a plurality of weft reserve turns and from which said turns unwind, when required, by the loom under the control of a braking element which ensures the necessary mechanical tension of the thread.
It is well-known to those skilled in the art that such weft feeders have a movement device suitable to move the turns wound by the windmilling arm from the base of the drum to its free end; the device also mutually spaces the turns by an extent, or spacing pitch, which can be changed within certain limits in order to pack or spread out the turns to correspondingly vary the weft reserve.
This conventional device is constituted by a set of movement rods which partially and cyclically protrude from corresponding slots of the drum and are subjected to a movement system which is suitable to give them a substantially undulatory motion, through which all the turns are cyclically lifted from the surface of the drum and deposited back onto it after they have been moved forward longitudinally, towards the free end of the drum, by an extent which is equal to the spacing pitch.
The movement rods, which are parallel to the generatrices of the drum, are individually connected, by means of corresponding spokes and in a swift-like configuration, to a central hub which is rotationally coupled, with the interposition of a rolling bearing, so that the movement swift is rotationally fixed, to a bush whose axis is oblique with respect to the axis of the driving shaft of the weft feeder. More specifically, the bush is fitted on an eccentric portion of the driving shaft or on an eccentric sleeve which is keyed on the shaft and in any case forms an eccentric axis, and has an axial hole which is tilted at a preset angle .alpha. with respect to the eccentric driving axis.
From the prior utility model no. 198.379, which relates to a system of this kind, it is known to vary through 180.degree. the angular position of the bush with respect to the eccentric sleeve (or, in an equivalent manner, with respect to the eccentric portion of the driving shaft) in order to allow the advancement of the turns of the weft reserve for both directions of rotation S, Z (counterclockwise and clockwise) of the driving shaft that turns the windmilling winder arm; this is required in order to adapt to the direction of the twisting of the fed weft thread in order to avoid untwisting its fibers, especially during unwinding from the drum of the feeder.
In order to vary the spacing pitch of the turns, it is also known to vary the angle .alpha. of inclination of the bush, as described in Italian patent no. 1,204,330, or the angle .beta. at which the bush is keyed onto the eccentric portion of the driving shaft with respect to a reference plane, which is chosen so that it coincides with the diametrical plane that contains both the driving axis and the eccentric axis.
In known systems of the second type, the arrangement of the bush with respect to the driving shaft is such that--as described and illustrated hereinafter with reference to FIG. 4--in the configuration in which the keying angle is zero, the axis of the bush intersects the rotation axis of the driving shaft in a point which is adjacent to the base of the drum of the feeder and is located--relative to the advancement direction of the thread--upstream of the diametrical plane that contains the axes of the spokes of the movement rods. With this conventional arrangement, the adjustment range of the keying angle .beta., which determines the spacing pitch between the turns, is between -90.degree. and +90.degree. with respect to the reference plane .beta.=0 and the spacing pitch becomes zero for a corresponding nil value of the keying angle.
In conventional systems of the first kind too, the spacing pitch becomes zero when the angle .alpha. of the bush becomes zero. This is a considerable functional drawback, which forces one to act on the bush to vary its inclination or respectively its keying angle whenever the direction of rotation of the driving shaft is changed in order to adapt it to the direction of the twisting of the thread being processed.
In particular, in conventional devices of the second type, if a keying angle of for example +15.degree. is set for counterclockwise rotation S, producing a spacing pitch which has a preset value, when the direction of rotation Z is assumed it is necessary, in order to keep the spacing pitch unchanged, to turn the bush symmetrically with respect to the reference plane, so that the keying angle is .beta.=-15.degree..